Generally, barium magnesium aluminate (BAM; [(Ba, Eu2+)MgAl10O17]) phosphors are used in CCFLs (Cold Cathode Fluorescent Lamps) or EEFLs (External Electrode Fluorescent Lamps) for BLUs (Back Light Units) of LCDs or three-wavelength fluorescent lamps.
Unlike other phosphors (red-Y2O3:Eu or green-(La,Ce)PO4:Tb) used for three-wavelength lamps (CCFLs or EEFLs), the BAM phosphor suffers because deterioration, such as low luminous properties, severely occurs in the process of burning an organic material serving as a binder (Binder Burn-Out Step=BBO Step) at 700˜750° C. upon heat treatment for the fabrication of actual application products, for example, fluorescent lamps, CCFLs, or EEFLs. The deterioration of the BAM phosphor is known as the main reason to cause the deterioration of luminous properties of the phosphors generated under gas discharge during the use of the application product. In particular, it is known that the deterioration (change in color coordinates) of luminous properties of the BAM phosphor greatly affects the change in color coordinates of the three-wavelength lamp.
Thus, depending on the deterioration of the phosphor, the brightness and color coordinates of the lamp are gradually decreased compared to initial lighting. Moreover, the change in color coordinates undesirably results in varied color temperature of the lamp, therefore decreasing performance of display products.
Hence, in order to minimize the deterioration of the BAM phosphor itself, thorough research has been conducted as follows.
Japanese Patent Laid-open Publication No. 2003-82345 discloses a method of oxidizing part of Eu2+ to Eu3+ without the use of additional compounds, and a method of forming an oxide film or a fluoride film with the addition of Al, Si or La. Japanese Patent Laid-open Publication No. 2003-82344 discloses a method of increasing positive charges through substitution of Al or Mg with a tetravalent element (Ti, Zr, Hf, Si, Sn, Ge, or Ce), in order to prevent deterioration of the phosphor. Further, Japanese Patent Laid-open Publication No. 2003-382343 discloses a method of coating BAM with oxide, such as SiO2, Al2O3, ZnO, MgAl2O4, Ln2O3, LaPO4 or Zn2SiO4, or fluoride such as Si(OF)4, La(OF)3, or Al(OF)3 and then heat treating such BAM at 300˜600° C. in air so as to prevent adsorption of water or carbon dioxide due to the lack of oxygen present in the conductive layer of BAM.
Further, Japanese Patent Laid-open Publication No. 2002-348570 discloses a technique for heat treating a BAM phosphor containing silicon at 500˜800° C. in air so as to prevent deterioration of the phosphor under vacuum UV light. Korean Patent Laid-open Publication No. 2003-14919 discloses a technique for surface treating only a selective portion of the surface of a phosphor, thereby minimizing a decrease in performance due to coating. Korean Patent Laid-open Publication No. 2002-0025483 discloses a technique for continuously coating the surface of a BAM phosphor with SiO2 to a thickness of 5˜40 nm to cover the surface of the phosphor such that the phosphor does not deteriorate. U.S. Pat. No. 5,998,047 discloses a technique for coating the surface of a BAM phosphor with catena polyphosphates such that the phosphor does not deteriorate due to UV light. Japanese Patent Laid-open Publication No. 2000-303065 discloses a technique for coating a BAM phosphor, which is a phosphor for use in vacuum UV light, with borates, phosphates, silicates, halogens, nitrates, sulfates and carbonates containing Ba or Sr as a cation in order to prevent the thermal degradation of the phosphor. Also, Japanese Patent Laid-open Publication No. 2002-080843 discloses a technique for coating a first BAM phosphor with second phosphor powder that emits UV light, to prevent the decrease in performance of the first phosphor.
In addition, research into improvement of the properties of phosphor for increasing the ability to maintain brightness of a lamp as a specific application product example is as follows.
According to Japanese Patent Laid-open Publication Nos. Hei. 11-172244 and 9-231944, Japanese Patent Laid-open Publication Nos. 2002-348570, 2003-147350, 2003-226872, and 2004-244604, when the surface of a phosphor is treated with nitric acid and metal oxide, such as La2O3, Y2O3, SiO2, or Gd2O3, to form a 5-100 nm thick of rare earth metal oxide film on the surfaces of the phosphor particles (Japanese Patent Laid-open Publication No. Hei. 11-172244), or when the surface of a phosphor is coated with carbonates of rare earth metal (Japanese Patent Laid-open Publication Nos. 2003-147350, 2003-226872, 2004-244604), deterioration of brightness due to vacuum UV light was reported to decrease. However, the above patents reported only decreased degree of brightness and not a decrease in initial brightness itself of the phosphor due to coating, and also do not mention ability to maintain color coordinates.
In the case where the protective film is formed on the surface of the phosphor, the luminous efficiency varies with the thickness of the film. When the surface treatment amount is large, the decrease in efficiency is also large but the ability to maintain brightness is good. Further, the surface treatment material may entail a negative result of coagulating phosphor particles due to its action as a binder, as well as the positive function as a protective film. The phosphor thus coagulated exhibits poor dispensability upon actual use and therefore cannot be formed into a uniform coating film, undesirably leading to non-uniform color coordinates or brightness.